Abstract

We report on Hubble Space Telescope Faint Object Spectrograph observations of the double QSO HS 1216+5032 AB (z(e)(A) = 1.455; z(e)(B) = 1.451, and angular separation theta = 9. 1). The spectral coverage is 910 Angstrom to 1340 Angstrom in the QSO rest-frame. An unusual broad-absorption-line (BAL) system is observed only in the B component: maximum outflow velocity of similar to 5 000 km s(-1); probably a mixture of broad and narrow components. Observed ions are: H I, C II, C III, N III, N V, O VI, and possibly S IV and S VI. We also discuss two outstanding intervening systems: (1) a complex C Iv system at z = 0.72 of similar strength in A and B, with a velocity span of 1500 km s(-1) along the lines of sight (LOSs; LOS separation: S-perpendicular to[z = 0.72] approximate to 75 h(50)(-1) kpc); and (2) a possible strong Mg II system at z = 0.04 observed in B only, presumably arising in a damped Ly alpha: system. We assume HS 1216+5032 is a binary QSO but discuss the possibility of a gravitational lens system. The size of Ly alpha forest clouds is constrained using S-perpendicular to approximate to 80 h(50)(-1) kpc at redshifts between 2 = 1.15 and 1.45. Four Lya systems not associated with metal lines and producing lines with W-0 > 0.17 A are observed in both spectra, while five appear in only one spectrum. This sample, although scarce due to the redshift path blocked out by the BALs in B, allows us to place upper limits on the transverse cloud sizes. Modelling the absorbers as nonevolving spheres, a maximum-likelihood analysis yields a most probable cloud diameter D = 256 h(50)(-1) kpc and 2 sigma bounds of 172 < D < 896 h(50)(-1) kpc. If the clouds are modelled as filamentary structures, the same analysis yields lower transverse dimensions by a factor of two. Independently of the maximum-likelihood approach, the equivalent width differences provide evidence for coherent structures. The suggestion that the size of Ly alpha forest clouds increases with decreasing redshift is not confirmed. Finally, we discuss two z(a) approximate to z(e) Ly alpha systems observed in both QSO spectra.